AP1701/2/3/4

3-PIN MICROPROCESSOR RESET CIRCUITS

AP1701/2/3/4 Rev. 2

4 of 9

FEBRUARY 2007

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Function Description

A microprocessor’s (µP’s) reset input starts the µP in a

known state. The AP1701/2/3/4 assert reset to prevent

code-execution errors during power-up, power-down, or

supply voltage declines below a preset threshold, keeping it

threshold. The AP1701/2/3/4 have a push-pull output stage.

Applications Information

In addition to issuing a reset to the µP during power-up,

power-down, and brownout conditions, the AP1701/2/3/4 are

(glitches).

The AP1701/2/3/4 do not generate a reset pulse. The

starting 0.5V above the actual reset threshold and ending below it

by the magnitude indicated (reset comparator overdrive). The

transient can have without causing a reset pulse. As the

magnitude of the transient increases (goes farther below the

reset threshold), the maximum allowable pulse width decreases.

threshold and lasts 100µs or less will not cause a reset pulse. A

pin provides additional transient immunity.

Ensuring a Valid Reset Output

below the reset threshold),

CC

goes below the reset threshold, the internal timer resets to zero,

and

CC

returns above the reset threshold, and

for the reset timeout period.

RESET

output no longer sinks current—it becomes an open circuit.

Therefore, high-impedance CMOS logic inputs connected to

This presents no problem in most applications since most µP and

applications where

pull down resistor to

currents to flow to ground, holding

not critical; 100k are large enough not to load

small enough to pull

Benefits of Highly Accurate Reset Threshold

Most µP supervisor ICs has reset threshold voltages

between 5% and 10% below the value of nominal supply voltages.

This ensures a reset will not occur within 5% of the nominal

supply, but will occur when the supply is 10% below nominal.

When using ICs rated at only the nominal supply ±5%, this leaves

a zone of uncertainty where the supply is between 5% and 10%

low, and where the reset may or may not be asserted.

RESET

Block Diagram

Delay Circuit

Driver

RESET

/ RESET